Here's a summary of a new study which shows that guys who smelled t-shirts of ovulating girls had higher testosterone levels than those who smelled unworn t-shirts or those worn by non-ovulating girls. You can find the full article free here. A guy's testosterone level rose as the girl who'd worn the t-shirt came closer to ovulation, and the level fell as she passed ovulation. The same rising-and-falling pattern in tandem with time to ovulation was seen for the guy's subjective evaluation of how pleasant it smelled.
More news you can use: keep your T-level from falling by making sure there are lots of young girls around you. Those used in the study were 18-19 for one experiment, and 18-21 for another. During the mid-20s, these olfactory cues nearly shut off, which makes sense given that she's not expected to be in the mating market anymore but rather to be raising her kids. No science there other than my own experience and the power of my sense of smell (and supporting theory).
Because evolutionary psychology is still a sexy new discipline, many older results bearing on their topics are just not well known. This happens in academia in general because theories go through fashion cycles, and competitive pressures to find out the truth aren't as strong as they should be. The list of references of the above article hardly has anything before the late '80s, which is when evolutionary psychology started marketing itself as something different from sociobiology.
Unfortunately that means the authors missed this key supporting study from way, way back in 1975 -- although it was published in Science, one of the two major academic journals, along with Nature. In brief, they found that men rated the scent of vaginal secretions from the ovulatory phase of a girl's menstrual cycle as better than those from other phases. Clearly there aren't enough well-read perverts among the peer reviewers for Psychological Science.
If evolutionary psychologists were as daring as they make themselves out to be, they'd re-establish the neglected "wet panties" experiment rather than crank out yet another "sweaty t-shirt" experiment.